In an internal combustion engine, the friction existing between the cylinder liner and the piston rings is widely studied because the reduction thereof is crucial to optimise the functioning of engines, improving performance, reducing the consumption of fuel and decreasing the burning of lubricating oil present on the cylinder wall.
A paradigm current until very recently was that wherein the improvement was sought of each component individually. Consequently, research workers developed new improvements (such as, for example, base materials and coatings) for rings and cylinder liners, in terms of the properties of each one thereof. A systematic study having the objective of optimising the performance of more than one component of the combustion cell was uncommon.
Consequently, in many situations the development of a piston ring having a certain coating being very efficient in terms of wear did not provide economic benefits by virtue of the fact that the said new coating did not perform satisfactorily in the cylinder liners in use.
The great majority of cylinder liners currently in existence are produced in cast iron, a material with excellent properties in terms of tribological compatibility with the rings and the skirts of the piston. This compatibility arises from the fact that cast iron has, naturally, particles of free carbon (graphite) which, by virtue of being a solid lubricant, contributes to the reduction of dynamic friction and, by inference, to the greater durability of the said components.
Recently, with the increase in pressure for a reduction in consumption and pollution by engines brought about by increasingly restrictive pollutant control legislation, engine designers have made use of solutions such as supercharging at increasingly high pressures and new strategies for pollution reduction, such as the utilisation of exhaust gas recirculation (EGR) systems.
These solutions have taken the liners of cast iron to what appears to be the limit of tribological application by virtue of the fact that the said components have demonstrated accelerated wear in these new engines, in particular in the region of the top dead centre (TDC).
As a consequence, the aforementioned increase in pressures to reduce consumption and pollution by engines, as a result of increasingly restrictive pollutant control legislation, has brought about the need for a paradigm change and manufacturers have started thinking about the joint development of rings and liners in a systematic manner, going on to develop new sliding pairs.
A first fruit of this joint approach may be found in international patent application WO2009/069703 which refers to the combined structure of a piston ring and a cylinder liner of an internal combustion engine. The combined structure comprises: (i) a cylinder liner, the sliding surface whereof has an average roughness, taken at 10 different points, of 0.5 μm to 1.0 μm, an effective load roughness (Rk) of 0.2 μm to 0.4 μm, an initial wear height (Rpk) of between 0.05 μm and 0.1 μm, and an oil sump depth (Rvk) of between 0.08 μm and 0.2 μm; and (ii) a piston ring, the external lateral face whereof has an average roughness, taken at 10 different points, of 1.6 μm or less, and an initial wear height of less than 0.3 μm. Furthermore, the piston ring should exert a pressure on the cylinder liner of the order of 0.03 MPa to 0.2 MPa.
The patent document JP2004116707 refers to a combined sliding member formed by a piston ring and a cylinder liner. The piston ring comprises a sliding surface having a roughness Rz of 0.5 μm to 1.0 μm and the cylinder liner has a roughness Rz of 0.5 μm to 1.5 μm, an initial wear height of 0.05 μm to 0.2 μm (based on the standard DIN 4776), an effective load roughness (Rk) of 0.2 μm to 0.6 μm, and an oil sump depth (Rvk) of 0.10 μm to 0.35 μm.
The external lateral surface of the piston ring is formed of a layer of metallic chromium, a laminated layer of metallic chromium, a nitrided layer or a layer applied by the physical vapour deposition (PVD) process. The surface of the cylinder liner, in turn, is preferably formed of cast iron, cast iron with boron or cast steel.
Finally, the Japanese patent document JP2003254156 refers to the combination of a cylinder and a piston ring, wherein there occurs a low degree of sliding friction, reduced scuffing wear, and a reduction in the loss of the combustion gases through the passage between the rings and the sliding surface of the liner and burning lubricating oil (blow by). The cylinder is made from an alloy based on aluminium and the piston ring has a base of austenitic stainless steel, the external lateral surface being coated by a film of hard chromium constituted by two or more layers wherein microcracks are distributed, each thereof including a part which opens at the external surface and has a depth corresponding to the layer itself remaining and extending partially in the direction of a deeper layer. This constitution ensures an excellent degree of oil retention in these microcracks, ensuring the durability and reducing the consumption of lubricant through blow by.
There had still not been developed a sliding combination achieving the desired gains of durability and strength through the use of a ring with a metal nitride coating applied by the physical vapour deposition (PVD) process and a cylinder liner with a DLC (diamond-like carbon) coating. Studies carried out by the applicant demonstrate that the utilisation of other types of coating on the piston ring, such as chromium ceramic or chromium nitride applied by the thermospray process did not provide the expected results.